on the medium land. Their relative value must be in proportion to the cost on that quality of land, whatever it may be, on which the comparative demand for the two grains requires that both of them should be grown. If, from the state of the demand, the two cultivations meet on land more favourable to one than to the other, that one will be cheaper and the other dearer, in relation to each other and to things in general, than if the proportional demand were as we at first supposed.
Here, then, we obtain a fresh illustration, in a somewhat different manner, of the operation of demand, not as an occasional disturber of value, but as a permanent regulator of it, conjoined with, or supplementary to, cost of production.
The case of rotation of crops does not require separate analysis, being a case of joint cost of production, like that of gas and coke. If it were the practice to grow white and green crops on all lands in alternate years, the one being necessary as much for the sake of the other as for its own sake; the farmer would derive his remuneration for two years' expenses from one white and one green crop, and the prices of the two would so adjust themselves as to create a demand which would carry off an equal breadth of white and of green crops.
There would be little difficulty in finding other anomalous cases of value, which it might be a useful exercise to resolve: but it is neither desirable nor possible, in a work like the present, to enter more into details than is necessary for the elucidation of principles. I now therefore proceed to the only part of the general theory of exchange which has not yet been touched upon, that of International Exchanges, or to speak more generally, exchanges between distant places.
